I'd advise against clerks, honestly. The term "clerk" can mean anything you want it to mean. As Yaroslav pointed out, in a legal setting judicial and court clerks can be quite influential. In government administration, at least in the U.S., a clerk is often an elected position with responsibilities delineated by statute. Because there's no commonly understood role, it's subject to creep. And filling the role with a volunteer has a lot of potential pitfalls.
I agree with Thomas and Risker that it's likely administrative support for the FDC will be necessary... but I think it makes more sense to have it be professional, paid administrative support of a limited capacity. Scheduling, note-taking, travel arrangements, etc. If there is a concern about having a WMF paid staff member closely involved with the FDC, allocate the FDC a specific budget (including other costs, such as travel) and allow the committee to do the hiring.
Unlike Risker, I think a lot of what the FDC will do is already fairly well known. Nothing is set in stone, but to me it sounds like the unknowns revolve around the composition of the committee and how it will decide to distribute funds. We know it will be a committee of people, with meetings both in person and virtual, that makes funding decisions for Wikimedia; as such, its administrative needs really aren't that opaque. It should be roughly similar to what the board of trustees needs now.
Nathan